1. Field of the Invention
The present invention is directed to phosphor powders, methods for producing phosphor powders and devices incorporating the powders. In particular, the present invention is directed to sulfur-containing phosphor powders having small average particle size, a narrow particle size distribution, high crystallinity and a spherical morphology. The present invention also relates to a method for continuously producing such sulfur-containing phosphor powders and to devices that incorporate the powders, such as flat panel display devices.
2. Description of Related Art
Phosphors are compounds that are capable of emitting useful quantities of radiation in the visible and/or ultraviolet spectrums upon excitation of the phosphor compound by an external energy source. Due to this property, phosphor compounds have long been utilized in cathode ray tube (CRT) display devices, such as televisions, computers and similar devices. Typically, inorganic phosphor compounds include a host material doped with a small amount of an activator ion.
More recently, phosphor compounds, including phosphors in particulate form, have been utilized in many advanced display devices that provide illuminated text, graphics or video output. In particular, there has been significant growth in the field of flat panel display devices such as liquid crystal displays, plasma displays, thick film and thin film electroluminescent displays and field emission displays.
Liquid crystal displays (LCD) use a low power electric field to modify a light path and are commonly used in wristwatches, pocket televisions, gas pumps, pagers and similar devices. Active matrix liquid crystal displays (AMLCD) are commonly used for laptop computers. Plasma display panels (PDP) utilize a gas, trapped between transparent layers, that emits ultraviolet light when excited by an electric field. The ultraviolet light stimulates phosphors on the screen to emit visible light. Plasma displays are particularly useful for larger displays, such as greater than about 20 diagonal inches. Thin film and thick film electroluminescent displays (TFEL) utilize a film of phosphorescent material trapped between glass plates and electrodes to emit light in an electric field. Such displays are typically used in commercial transportation vehicles, factory floors and emergency rooms. Field emission displays (FED) are similar in principle to CRT's, wherein electrons emitted from a tip excite phosphors, which then emit light of a preselected color. Phosphor powders are also utilized in electroluminescent lamps (EL), which include phosphor powder deposited on a polymer substrate which emits light when an electric field is applied.
There are a number of requirements for phosphor powders, which can vary dependent upon the specific application of the powder. Generally, phosphor powders should have one or more of the following properties: high purity; high crystallinity; small particle size; narrow particle size distribution; spherical morphology; controlled surface chemistry; homogenous distribution of the activator ion; good dispersibility; and low porosity. The proper combination of the foregoing properties will result in a phosphor powder with high luminescent intensity and long lifetime that can be used in many applications. It is also advantageous for many applications to provide phosphor powders that are surface passivated or coated, such as with a thin, uniform dielectric or semiconducting coating.
Numerous methods have been proposed for producing sulfur-containing phosphor particles. One such method is referred to as the solid-state method. In this process, solid phosphor precursor materials are mixed and are heated so that the precursors react in the solid-state and form a powder of the phosphor material. It is difficult to produce a uniform and homogenous phosphor powder by solid state methods. Further, solid-state routes, and many other production methods, utilize a grinding step to reduce the particle size of the powders. Mechanical grinding damages the surface of the phosphor, forming dead layers which inhibit the brightness of the phosphor powders.
Phosphor powders have also been made by liquid precipitation methods. In these methods, a solution which includes phosphor particle precursors is chemically treated to precipitate phosphor particles or phosphor particle precursors. The precipitated compounds are typically calcined at an elevated temperature to produce the final phosphor material. An example of this type of preparation is disclosed in U.S. Pat. No. 5,413,736 by Nishisu et al. In yet another method, phosphor particle precursors or phosphor particles are dispersed in a solution which is then spray dried to evaporate the liquid. The phosphor particles are thereafter sintered in the solid state at an elevated temperature to crystallize the powder and form the phosphor compound. Such a process is exemplified by U.S. Pat. No. 4,948,527 by Ritsko et al. and U.S. Pat. No. 3,709,826 by Pitt et al.
International Application No. PCT/US95/07869 by Kane discloses a process for preparing oxysulfide phosphor particles having a particle size of 1 .mu.m or less that are spherical in shape. In this process, hydroxycarbonate compounds are precipitated from solution. The hydroxycarbonates are then heated in oxygen to form an oxide which is then heated in a sulfur-containing flux to form the oxysulfide phosphor.
U.S. Pat. No. 3,676,358 discloses a process wherein a solution of precursor nitrates are atomized and heated at 400.degree. F. to dry the particles. The particles are then passed through a flame to react and form the phosphor.
Tohge et al. in an article entitled "Formation of Fine Particles of Zinc Sulfide from Thiourea Complexes by Spray Pyrolysis" (Japanese Journal of Applied Physics, Vol. 34, 1995, pgs. 207-209) disclose particles of ZnS fabricated by ultrasonic spray pyrolysis of an aqueous solution. The particles are spherical with a smooth surface and have a size range of from 0.5 to 1.3 .mu.m. It is disclosed that particles reacted at 400.degree. C. are amorphous whereas particles reacted at 600.degree. C. and higher show crystalline phases. Partial oxidation of the zinc sulfide above 900.degree. C. was also observed. Tohge et al. have also disclosed the formation of cadmium sulfide by a similar process in an article entitled "Formation of CdS fine particles by spray-pyrolysis" (Journal Material Science Letter, Vol. 14, 1995, pgs. 1388-1390).
Despite the foregoing, there remains a need for phosphor powder batches that include particles having a small size, substantially spherical morphology, narrow particle size distribution, a high degree of crystallinity and good homogeneity, which result in high luminescent intensity. The powder should have good dispersibility and the ability to be fabricated into thin layers having uniform thickness, resulting in a device with high brightness.